Let push-to-trunk script retry failing svn writes.

BUG=
TEST=python -m unittest test_scripts
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/144463003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18771 ce2b1a6d-e550-0410-aec6-3dcde31c8c00